Enforce canonical php-transformer result contracts

[chubes4]

Summary

• Remove legacy_mapping from the canonical php-transformer result envelope.
• Add TransformerResult::assertCanonicalEnvelope() so downstream wrappers can require the canonical source_reports.conversion_report contract and artifact source_reports.materialization_plan contract.
• Make FormatBridge success and failure results emit canonical source_reports.conversion_report metadata.
• Add contract coverage rejecting legacy aliases and missing canonical reports/plans.

Verification

• composer validate from php-transformer/
• composer test from php-transformer/
• git diff --check

Downstream impact

• Wrappers can fail fast on non-canonical result envelopes instead of accepting BAC/BFB/H2BC-shaped aliases.
• Artifact consumers can require source_reports.materialization_plan as the stable materialization contract.
• Compatibility-only legacy projection remains downstream-owned rather than part of Blocks Engine's main result path.

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Implementing canonical result contract enforcement, updating tests, and running verification.